Many industrial processes, especially chemical processes, operate at elevated pressures. These processes require a high pressure fluid feed, which may be a gas, a liquid or a slurry, to produce a fluid product or effluent. One way of providing a high pressure fluid feed to such an industrial process is by feeding a relatively low pressure stream through a pressure exchanger to exchange pressure between a high pressure waste stream and the low pressure feed stream. One particularly efficient type of pressure exchanger is a rotary pressure exchanger wherein a rotating rotor having axial channels establishes hydraulic communication between the high pressure fluid and the low pressure fluid in alternating sequences.
U.S. Pat. Nos. 4,887,942; 5,338,158; 6,537,035; 6,540,487; 6,659,731; and 6,773,226, the disclosures of which are incorporated herein by reference, discuss rotary pressure exchangers of the general type described herein for transferring pressure energy from one fluid to another. This type of pressure exchanger is a direct application of Pascal's Law: “Pressure applied to an enclosed fluid is transmitted undiminished to every portion of the fluid and to the walls of the containing vessel.” Pascal's Law holds that, if a high pressure fluid is brought into hydraulic contact with a low pressure fluid, the pressure of the high pressure fluid is reduced, the pressure of the low pressure fluid is increased, and such pressure exchange is accomplished with minimum mixing. A rotary pressure exchanger of this type applies Pascal's Law by alternately and sequentially bringing a channel which contains one lower pressure fluid into hydraulic contact with another higher pressure fluid thereby pressurizing the one fluid in the channel and causing some fluid that was in the channel to exit to the extent that higher pressure fluid takes its place, and thereafter bringing the channel into hydraulic contact with a second chamber containing the incoming stream of lower pressure fluid which pressurizes the fluid in the chamber sufficiently to cause some of the other fluid in the channel to exit at still lower pressure.
The net result of the pressure exchange process, in accordance with Pascal's Law, is to cause the pressures of the two fluids to approach one another. The result is that, in a chemical process, such as sea water reverse osmosis, for example, operating at high pressures, e.g., 700-1200 pounds per square inch (psi), where a seawater feed is generally available at low pressures, e.g., atmospheric pressure to about 50 psi, and a high pressure brine from the process is available at about 700-1200 psi, the low pressure seawater and the high pressure brine can both be fed to such a pressure exchanger to advantageously pressurize seawater and depressurize waste brine. The advantageous applicable effect of the pressure exchanger on such an industrial process is the reduction of high pressure pumping capacity needed to raise the feed stream to the high pressure desired for efficient operation, and this can often result in an energy reduction of up to 65% for such a process and a corresponding reduction in required pump size.
In such a rotary pressure exchanger, there is generally a rotor with a plurality of open-ended channels. Rotation of the rotor is driven either by an external force or by the directional entry of the high pressure fluid into the channels, as known in this art. Rotation provides alternating hydraulic communication of the fluid in one channel exclusively with an incoming high pressure fluid in one of the opposite end chambers and then, a very short interval later, exclusively with an incoming low pressure fluid in the other end chamber. As a result, axially countercurrent flow of fluid is alternately effected in each channel of the rotor, creating two discharge streams, for example a reduced pressure brine stream and an increased pressure seawater stream.
In such a rotary pressure exchanger having a rotating rotor with a plurality of substantially longitudinal channels extending through the rotor, there will be many very brief intervals of hydraulic communication through between chambers at the opposite ends holding the two fluids which are otherwise hydraulically isolated from each other. Minimal mixing will occur in the channels because operation is such that the channels will each have a zone of relatively dead fluid that serves as a buffer or interface in that channel between the fluids which enter and exit from one respective end. This permits the high pressure brine to transfer its pressure to the incoming low pressure seawater stream without mixing.
The rotor usually rotates in a cylindrical sleeve or housing, with its flat end faces slidingly and sealingly interfacing with end cover plates. These end covers are peripherally supported by contact with the sleeve and have separate inlet and discharge openings for alternately mating with the channels in the rotor. As a result, these channels alternately hydraulically connect with, for example, an incoming high pressure brine stream and then with an incoming low pressure seawater stream; in both instances, there is discharge of liquid from the opposite end of the channel. As the rotor rotates between these intervals of alternate hydraulic communication, channels are briefly sealed off from communication from both openings in each of the end covers.
The rotor in the pressure exchanger is often supported by a hydrostatic bearing and driven by either the flow of fluids into and through the rotor channels or by a motor. To achieve extremely low friction, such a pressure exchanger usually does not use rotating seals, but instead, fluid seals and fluid bearings are used. Extremely close tolerance fits are used to minimize leakage.
To minimize such leakage and to improve the dimensional stability of constructional materials, improvements in rotary pressure exchangers of this type are continually being sought.